2009/4/14 aditya siram
<aditya.siram@gmail.com>
Hi all,
I am trying to understand the TCP Syslog Server example [1] from RWH.
Specifically my question is on this piece of code:
-- | Process incoming messages
procMessages :: MVar () -> Socket -> SockAddr -> IO ()
procMessages lock connsock clientaddr =
do connhdl <- socketToHandle connsock ReadMode
hSetBuffering connhdl LineBuffering
messages <- hGetContents connhdl
mapM_ (handle lock clientaddr) (lines messages)
hClose connhdl
handle lock clientaddr
"syslogtcpserver.hs: client disconnected"
How does control stay on "mapM_ (handle lock clientaddr) (lines
messages)" ? It would seem that the server would handle one message
and immediately close the handle and end - but it doesn't. I'm
guessing this has something to do with laziness, but I don't see how
to apply it.
I think you are right thinking about laziness. The function hGetContents produces a lazy
result (messages), therefore any function that consumes messages could be considered as 'iteratively calling' hGetContents to get new elements of messages ...
Laziness is a Very Cool Feature ... when it does not bite you with thunk accumulation problems :-)
Ciao
------
FB
thanks ..
-deech
[1] http://book.realworldhaskell.org/read/sockets-and-syslog.html
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